1. Field of the Invention
The present invention relates to a method of and an apparatus for coating the surfaces of powder particles with a liquid, for example, for coating corn starch powder particles with an aqueous solution of vitamin or the like.
2. Description of the Related Art
It is generally known that various kinds of surface-improving processes are utilized in many fields. For example, in a typical surface-improving process, particles of fine powder having low fluidity are coated with a substance having low adhesiveness for the purposes of improving the fluidity of the fine powder particles and hence the handling thereof. In another surface-improving process, a liquid material is deposited on solid particles so that a particular kind of powder can be used as various powder materials having different compositions.
To achieve such surface improvement or to deposit liquids onto solids, various kinds of methods and apparatus for use in coating the surfaces of powder particles with a liquid have heretofore been proposed. A typical known method is to spray powder particles with a coating liquid from an upper position while fluidizing the powder particles by an air flow. Another known method is to mix powder particles with a liquid by agitating them with a blade means which is rotating at high speeds.
However, as is known, each of the above-described liquid coating methods and apparatus has the problem that if powder particles to be coated are as fine as, for example, 30 .mu.m or less, an appropriate coating cannot be achieved. In general, such fine powder particles have a strong tendency to aggregate and form secondary particles and even if they are floated and fluidized by, e.g., an air flow, they are apt to include a considerable percent of secondary particles derived from the aggregation of primary particles. For this reason, the primary particles which have aggregated and formed the secondary particles cannot be subjected to an appropriate surface coating, thus resulting in the above-described problem.
To solve the problems experienced with the conventional method and apparatus described above, the present inventors have directed intensive efforts to the development of a method and an apparatus both of which are capable of suitably depositing a coating liquid on the surfaces of fine powder particles which tend to easily form secondary particles due to aggregation.
In addition, in the art of coating powder particles, it is desired to handle various deposition states. For example, it may be desired to deposit a liquid onto the entire surfaces of powder particles, or it may be desired to deposit a liquid onto part of the surfaces of powder particles. Desired deposition states differ for individual applications which require powder particles. However, in the conventional coating apparatus, it has been impossible to prepare such various deposition states.